This invention herein described was made under a contract with the Department of the Army.
This invention relates to signal processing techniques and more particularly to a signal processor and method for detecting and discriminating between various targets which generate signals. A useful application of the invention is in the processing of seismically generated signals.
U.S. Pat. No. 3,696,369, assigned to the assignee of this invention, discloses a seismic signal processing circuit which distinguishes between various types of targets on the basis of their seismic signatures. For example, a burst or short pulse type of seismic signal characteristic of the footstep of a walking man is distinguished from a continuous seismic signal which is characteristic of a helicopter on the basis of differences in frequency and signal duration. Such a processor has been very effective in discriminating between targets and non-targets and so have contributed considerably to the detection of selected signals with minimum false alarms.
In order to further optimize the performance of processors including the one described in the foregoing patent, it is necessary to take into account different seismic conductivity at the various sites under surveillance. The attenuation of a seismic signal with increasing source-to-detector distance can vary over wide ranges at different locations. Attenuation may vary from about 6 db or less for each doubling of distance of 14 db or more for doubling of distance. The signal output of a typical geophone in response to a man walking at a distance of 30 feet can vary from about 10 microvolts at an insensitive site to one millivolt at an unusually sensitive site. To amplify these signals to a reasonable processing level of 2 volts requires an amplifier voltage gain of 106 db in one case and only 66 db in the other case.
The usual practice under these conditions is to manually set a gain control at the actual site to obtain optimum performance or if the site characteristics are known, the gain can be preset. Such field adjustment is not only tedious and time consuming but also requires the services of a relatively skilled technician, thereby complicating the installation procedure and further increasing its cost. Another alternative is to preset the gain to some compromise level such as 98 db and accept less detection range at poor sites and greater detection range at sensitive sites with the attendant risk of poorer false alarm rejection.